BEAT sriracha and egg yolks in small bowl until blended. Whisk in about 4 Tbsp. of the milk sauce to temper; return to remaining sauce in sauce pot. Cook 3 to 5 min. or until thickened. Add cheeses, reserving 4 oz. Gouda for topping (or 2 oz. for trial recipe); cook and stir until melted. (Sauce should be thick enough to coat back of spoon.)